Acousto-optic modulators have been employed to control the respective intensities of multiple color (red-R, green-G and blue-B) components of multi-color laser projector systems. This is typically accomplished by using a respective modulator for each color or wavelength, and then recombining each of the beams into a single output beam by means of dichroic beam combiners. It is necessary to employ monochromatic modulation, since a common acousto-optic modulator is insensitive to color or wavelength. Disadvantageously, the cost, size, complexity, and the number of alignments of the system components required to implement such a parallel system are fairly substantial.
A laser projector system design can be considerably simplified, if the modulator is capable of accepting an effectively collinear "white" polychromatic light beam (i.e. a beam containing a sufficient number of different wavelengths or colors to produce a full spectrum color perception to the human visual system), and is able to produce an effectively collinear output beam, the color composition of which is controllable, as by means of electrical signal inputs. Namely, the desired output color is to be defined by electronically controlling the intensity of each color or wavelength component of the "white" beam.
There have been several proposals to combine acousto-optic modulation of an RGB beamby means of a `pseudo-single` device. Specifically, each of a technical data sheet entitled "Acousto-optic Device Provides Polychromatic Images," by Optikon Corporation Ltd., 410 Conestoga Road, Waterloo, Ontario, Canada, N2L 4E2, and the U.S. Pat. No. to Vichon et al, No. 4,798,449, describes a polychromatic acousto-optic modulation system, diagrammatically illustrated in FIG. 1, in which individual red-R, green-G, and blue-B modulators 11R, 11G, 11B are coupled to respective transducers 12R, 12G and 12B mounted in series on a common face 13 of a single acousto-optic medium 14. In such an arrangement, the amount of crosstalk, i.e. the undesirable effect of having the modulation of a specific wavelength or color leak into or affect the other colors, is minimized by physically separating the modulating regions 15R, 15G, 15B for the respective colors. Among shortcomings of this arrangement are the fact that its size, cost and complexity are undesirably large, its operation is limited to only a few wavelengths (e.g. three for a currently available device) and it suffers from poor color convergence.
Another type of device, diagrammatically illustrated in FIG. 2, having a crystalline coordinate axis diagram shown in FIG. 2A, and described in an article by M. Gazalet et al, entitled "Independent acousto-optic modulation of the two wavelengths of a bichromatic light beam," Applied Optics, Vol. 23, No. 5, 1 Mar. 1984, pp. 674-681, relies upon the angular difference (separation) of each of a plurality of input color beams 21, 22 with respect to acousto-optic medium 23, for each designated color (wavelength), in order to reduce inter-color modulation. This is accomplished as a result of a selective Bragg matching condition for an acoustic wave 25 launched by a transducer 27, mounted on a face 29 of acousto-optic medium 23.
In a conventional acousto-optic device, if the selectivity of the device with respect to wavelength is narrow for the purpose of restricting acousto-optic interaction with only a single color, two undesirable effects occur. First, the selectivity of the angular range of the optical beam also becomes narrow; secondly, the required transducer length (i.e. acousto-optic interaction length) becomes excessively large. The narrow angular range of the beam results in a distortion of the output or diffracted beam, while an increased transducer length may be impractical to implement. As a result, it is essential that angular separation of the colors at the input side 33 of the device be maintained.
It will be appreciated that criteria which dictate the design of conventional multi-color modulators, described above, restrict the number of selectable colors making up the output beam to only a few (two or three) and require the use of a relatively long transducer at a high operating frequency band. In addition, inter-color crosstalk is poor and, because of this custom design approach, the operational wavelengths must be specified in advance and match the operating parameters of the device, which makes the device configuration inflexible, so that it cannot be mass-produced for multiple applications.